1. Field of the Invention
The present invention relates to a method for forming high quality diamond films containing little amount of non-diamond components at low temperature by vapor phase synthesis, which are used as a novel material for semiconductors having a resistance against harsh environments and for optical coatings.
2. Description of the Related Art
For diamond growth by vapor phase synthesis, a reaction gas containing hydrogen and carbon is decomposed to deposit a diamond film on a substrate. Generally, the deposited film contains a diamond component and non-diamond components; however, the non-diamond components are etched by atomic hydrogen and thus the diamond component is predominantly deposited.
In a known technique of forming diamond films by direct current plasma chemical vapor deposition (CVD) using such an apparatus as shown in FIG. 4, a reaction gas is decomposed by a direct discharge plasma generated in an atmosphere of the reaction gas at approximately 200 Torr. This technique is disclosed in Journal of Institute of Electrical Engineers, vol. 106, No. 12, pp. 1211 (1986), and Japanese Patent Laid-open No. sho 63-85094.
In the apparatus as shown in FIG. 4, a reaction chamber 1 is evacuated by an exhaust pump (not shown) through an exhaust port 2, and is maintained at a specified vacuum. A disk cathode 3 is held from the upper wall of the chamber 1 through a bar shaft part 3aand a disk anode 4 is disposed on the bottom wall of the chamber 1 through a shaft part 4a. In the above, both the shaft parts 3a and 4a are electrically insulated from the chamber 1, and the cathode 3 and anode 4 are disposed oppositely in parallel to each other.
A reaction gas inlet 5 is disposed with the gas port thereof located near the cathode 3. A viewing port 6 is disposed at the central portion of the side wall of the chamber 1, through which the interior of the chamber 1 can be observed from the outside. The cathode 3 is connected to a negative power source 7, and the anode 4 is grounded through a current meter 8. A substrate 10 is placed on the anode 4 and is heated by a heater (not shown).
In operation, the substrate 10 is placed on the anode 4, after which the chamber 1 is evacuated through the exhaust port 2 and fed with a reaction gas containing hydrogen and carbon through the gas inlet 5. The substrate 10 is then heated at a temperature of approximately 600.degree. C. or more, and a direct current discharge is induced in an atmosphere of a reaction gas at approximately 200 Torr. Thus the reaction gas is decomposed and the diamond film is deposited on the substrate. In general, the deposited film contains a diamond component and non-diamond components; however, the non-diamond components are etched by atomic hydrogen and thus the diamond component is predominantly deposited.
As described above, in the conventional technique of forming diamond films by vapor phase synthesis, a substrate needs to be heated at a high temperature of 600.degree. C. or more. However, in such a high temperature range, substrates of low melting point metals such as aluminum, and III-V compound semiconducting materials such as gallium arsenide (GaAs) cannot be used. Namely, during the diamond film formation, the aluminum substrate is melted or deformed because of its low melting point; the GaAs substrate is changed in composition because of the evaporation of gallium; if silicon semiconductor devices are used as substrates for diamond overgrowth, they are damaged by the high temperature. Therefore, it is impossible to use the above materials as the substrate. Furthermore, when silicon semiconductor devices are hybridized with diamond films, the diamond growth temperature is restricted to be 450.degree. C. or less because of the poor thermal resistance. Accordingly, the diamond films, notwithstanding its excellent resistance against harsh environments, have been not used as protective films for silicon semiconductor devices or the like.